The present invention relates to a solid-state imaging device and, more particularly, to a solid-state image sensing device having a photoconductive film layer which is stacked to cover a photosensitive cell arrangement.
As a typical solid-state imaging device, a multi-layered CCD image sensor is known, wherein a photoconductive thin film is stacked on a normal CCD device, and an effective opening ratio of the image sensor is increased, whereby the utilization efficiency of incident image light is increased, so that the photoelectric sensitivity can be improved. With the image sensor of this type, since most incident image light is absorbed by the photoconductive thin film, the necessity of charge generation inside a chip substrate is reduced, and hence, a smear phenomenon inherent to the solid-state image sensor can be prevented.
However, the conventional multilayered CCD image sensor suffers from degradation in the reproduced image quality due to blooming and/or afterimage. More specifically, when the image sensor of this type is constituted by an interline transfer CCD image sensor, a signal charge capacity (i.e., a maximum amount to which signal charges generated upon irradiation of incident light can be accumulated therein) of a photoelectric transducer cell section is normally set to be larger than a capacity of a vertical charge transfer section or a horizontal charge transfer section, which is also known as a horizontal CCD shift register section (i.e., a maximum amount of signal charges to be transferred). When strong light is incident on the image sensor, excess signal charges (called "excess-charges") produced in the photoelectric transducer cell section in response thereto overflow while being transferred in the transfer section. For example, if one of the packets of signal charges of adjacent pixels overflows from a corresponding potential well formed in a substrate and is mixed with the other packet, this induces the blooming phenomenon or afterimage. As a result, the quality of a reproduced image is degraded.
According to Japanese Patent Disclosure (KOKAI) No. 140773/81, a technique is proposed wherein, in the CCD image sensor, excess charges in a transfer section are swept out to a CCD output section through a horizontal CCD shift register section upon application of a high-speed sweep-out pulse signal to vertical charge transfer sections in a vertical blanking period of the CCD image sensor. However, with this technique, the sweep-out of the excess charges is limited to a relatively short vertical blanking period. In addition, the excess charges are transferred to the output section by simply continuously applying a high-frequency pulse signal during the vertical blanking period, and are discharged therefrom. The excess charges to be swept out are transferred through the vertical charge transfer sections in the same direction as a transfer direction for reading out normal signal charges, and are discharged to the CCD output section. Therefore, a sweep out capability of the excess charges is limited, since it is difficult, due to limitations of the circuit design, to set the frequency of the sweep-out pulse signal too high. Therefore, it is difficult to realize a CCD image sensor having a wide dynamic range.
Japanese Patent Application No. 90416/84 teaches a technique wherein, in the CCD image sensor, after excess charges are transferred from a photoelectric transducing cell section to a vertical charge transfer section, the potential of the electrode of the vertical charge transfer section is forcibly set at a predetermined potential level, and the excess charges are averaged along the longitudinal direction of the vertical charge transfer section. Thereafter, the excess charges which are averaged along the longitudinal direction of the vertical charge transfer section are discharged to the output section through a horizontal CCD shift register section. With this technique, however, when excessively strong image light is radiated and too many excess charges are introduced to the vertical charge transfer section accordingly, charges overflow during transfer. Therefore, satisfactory improvement of blooming and afterimage cannot be expected. Therefore, it is difficult to realize a CCD image sensor with a wide dynamic range.